1. Field of the Invention
The invention relates to a method of making manholes and trenches in hard top traffic areas, that is, traffic lanes for track free vehicles, such as streets and parking lots which have a substructure and a top structure which includes a support layer and a top layer of bituminous material or concrete and to an apparatus for performing the method.
2. Description of the Prior Art
In cities and also in small communities the utility lines for water and sewage, for electricity and gas as well as low voltage services (telephone and cable TV) are now generally disposed underground, that is, under the streets or beside the streets. Even newly paved streets and sidewalks must therefore be cut open if a breakdown occurs in any of the utility lines in order for such lines to be repaired. Generally it is sufficient to excavate a manhole, that is, essentially a square cavity of no more than 1 m side length. For water supply and drainage pipes however it is usually necessary to excavate a larger area. It is also often necessary to excavate a trench for the repair of the damaged lines or the reinstallation of such lines.
To cut an opening for such a manhole or trench an air hammer or a hydraulic hammer with a chisel blade is generally used to break an appropriately sized area out of the hard top surface and the underground is excavated in the usual manner with pick ax and shovel or with an excavating machine. After repair of the damage the hole is backfilled up to the support layer and is compacted and is then finally filled with a material which should correspond to the original surface layer material. Generally however the result is quite disappointing. For one reason the cutting of the top layer by an air hammer generates rugged edges for the manhole or the trenches and for another reason it is general experience that, in spite of the compaction of the underground fill material, the ground including the top structure still settles so that, depending on the traffic load, the area of the manhole or the trench forms, within a period of half a year or a year, a noticeable depression which can be eliminated only by additional backfilling. But even after backfilling repairs, the underground generally further settles so that, after a certain time, the same depression reoccurs. The reason for this is that the whole hard top surface generally distributes the load and the underground support or load carrying layers compensate for underground deformations. With later backfilling the original conditions for the top and support layers cannot be restored since the substructure, even after compaction, can move down or sidewardly whereas the backfilled top layer is not firmly joined to the original top layer and therefore glides slowly downward together with the pavement.
In order to avoid the ugly rugged edges of the cutouts for the repair openings, circular saws are sometimes utilized for cutting the hard top layer and partially even the support layer before the jack hammer is used along the saw cut line. However while this provides for a neater repair it cannot prevent the settling of the backfill material since no stable connection between the backfill and the original material layers is achieved.
Recently it has been proposed to excavate a manhole or a trench to the desired width by means of a jack hammer and/or an excavator and, after backfilling and compacting the substructure, to cut out the top layer and part of the support layer at a distance from the original hole or trench walls by means of a circular saw cut and to remove the cut off material. In this way a stepped area is formed between the compacted substructure and the remaining part of the support layer which can be filled with support layer material and finally with top layer material. Although these measures reduce the settling of the substructure and sharp clean joints are produced, the backfill support layer material is not interlocked with the original support layer material because of the smooth saw cut which again enhances settling of the backfill material. Also, because of the deep circular saw blade cuts, tooling costs are relatively high.